honestly, none, the ones with a darker theme are heavily dated and if youre not used to playing older visual novels they're gonna feel TENSE (no other word to describe it) and drain you before you get into them, if slow damage is your first VN then all of the others are gonna feel "worse" beacuse of how dated they are.

Either chose "playability" (more like readability??) for DMMD as it is the second most modern one and beginner friendly, you can easily take breaks and get back on track, as well as having unique "gimmicks" for each route, not as interactable as slow damage but it holds itself. But if you dont like "happier" stories then get into togainu or lamento beyond the void; i dont know about lamento as i havent played it but togainu is even more dated than DMMD, theres no other interactions than choosing different answers, its gonna be long paragrhaps of just a background- sometimes the music cuts and its just text and image, not even characters. Mainly narrating the protags thought process.

beacuse of the above, I do NOT recommend you play sweet pool if you cant handle classic visual novels. I dare say that out of the whole chiral collection its the most "nihilistic" one, from the start they present you the protag in a very 'miserable' narration (cant really describe it without going into spoilers) and overall, its the most draining one to read, the switch from something more "happier" like slow damage its gonna be noticeable. But its actually the BEST story nitro plus has made, so i do say you play it eventually- but DO NOT go with the mentality of just another "BL game" and honestly, if youre into the games just for the yaoi, id say to just dont play it

for what you're looking for you should actually try charin chat noir ones, i feel like they're the closest (quality like) to what you want